1. Field of the Invention
The present invention relates to a method for the electrolytic removal of plastic flash or bleed formed on metal surfaces during plastic molding of semiconductor devices and the like by immersing such components in an aqueous (water-based) solution of one or more organic solvents, conducting salts, and a wetting agent and whereby the components to be cleaned are connected to the negative pole of a source of direct current of which the positive pole is connected to an anode in the same solution. The invention also relates to bath compositions to be used for immersing the components.
2. Discussion of the Background
In the manufacturing process of the majority of semiconductors, integrated circuits, transistors and the like for briefness's sake further called semiconductor components, the following production steps can be distinguished. A carrier, called a leadframe is stamped or etched from a copper alloy; nickel-iron alloy or an other metal. A leadframe comprises generally a pad and a number of leads.
The central part of the leadframe, consisting of pad and leadtips, is subsequently selectively electroplated with metals such as gold, silver, palladium, copper or nickel. In the next step a chip is bonded to the leadframe pad and electric wire connections are made between chip and leadtips (wirebonding), followed by encapsulating the central area containing the chip and wirebonded leadtips by injection molding with a plastic, usually an epoxy-based plastic. The manufacturing process is further explained by FIG. 1 in which an integrated circuit (IC) is shown manufactured as described above and of which the outer leads are contaminated with plastic bleed.
In FIG. 1, the plastic encapsulation, which contains the bonded chip and the wirebonded leadtips is indicated by numeral 1, the outer leads by 2. Numeral 3 indicates spaces which usually fill up with plastic during molding of encapsulation 1, while numeral 4 indicates places where very thin films of plastic (normally called "bleed") can contaminate the metal surface of the outer leads and which must be removed. Numeral 5 indicates parts of the leadframe serving interconnection during manufacturing and which will be removed before the IC is used.
The method of the present invention is designed to remove the plastic bleed as indicated in FIG. 1 by numeral 4. Such plastic bleed films on the metal surface of the leads are thin, usually less than 10 micrometers thick, but must be carefully removed to avoid uncoated areas during subsequent tin-lead plating; necessary to enable reliable solder connections to be made to other parts of an electronic circuit.
The traditional way to remove the plastic bleed films from the molded components is to submit the components to abrasive blasting with ground cherry or apricot stones, fine, relatively hard plastic grains or glass beads and the like. Sometimes the blasting operation is preceded by softening of the plastic bleed in organic solvents or by extended treatment in diluted warm acids. A disadvantage of blasting with fine particles is that some of these particles will be embedded in the metal surface of the leads and must be carefully removed before plating with a solderable coating and that the blasting operation will create roughness of the metal surface.
U.S. Pat. No. 4,968,397 describes an electrolytic anodic process with which embedded blasting particles can be removed, while simultaneously metal bleed, such as silver, extending on the leads outside the plastic encapsulation is dissolved, resulting in a clean surface of the leads. To accomplish this effect a solution of lactic acid and a metal hydroxide is used, in which the components are connected to the positive pole of a source of direct current and whereby simultaneously an improved surface condition of copper based metal leads is obtained.
U.S. Pat. No. 4,781,804 describes a method to remove plastic bleed from the leads of semiconductor components by means of an electrolytic treatment in a solution containing an alkali-metal hydroxide, detergent, a glycol compound, alkali metal bicarbonate, ethoxylated furfuryl alcohol, tetrahydrofurfuryl alcohol and a fluorinated surfactant in water and whereby the components are connected as a cathode to a source of direct current so that hydrogen is generated on the metal surface upon current passage. In practice, it appears that this solution contains aggressive components attacking the material of the plastic encapsulation.
Thus, there remains a need for an improved method for removing plastic flash and/or bleed from the metal surfaces of semiconductor devices and the like. There also remains a need for bath compositions to be used in such a method.